When an empty hash is splatted (using **) into a call of a method with no parameters, the empty hash is passed in as a positional argument instead of doing nothing. This causes an ArgumentError, which is confusing because when you splat an empty array into a method that doesn't accept any arguments the method is called without raising an error.

Similarly, if you splat a hash into a method that only has positional arguments, the method is called with the hash added as the last argument. This either causes an ArgumentError or unexpected bugs.

defwithout_parameters# some codeenddefwith_parameters(*args)argsenddefwith_one_parameter(arg)argendempty_hash={}filled_hash={example: "value"}array=[]without_parameters(*array)# calls the method without an error because `array' is emptywithout_parameters(**empty_hash)# unexpectedly raises an ArgumentError despite `empty_hash' being emptywith_parameters(**empty_hash)# unexpectedly returns [{}] instead of []with_parameters(**filled_hash)# unexpectedly returns [{ example: "value }] instead of raising an ArgumentErrorwith_one_parameter(**empty_hash)with_one_parameter(**filled_hash)# both unexpectedly do not raise an ArgumentError

This behavior makes it more difficult to do things like write specialized decorator classes using #method_missing. The following example does not work if the method being called does not have any named parameters. The variable named_args gets passed in as a positional argument, causing ArgumentErrors or unexpected bugs:

More details about why errors are not raised in some cases, and wrong errors are raised in other cases:

defexample(required_param,**optional_named)prequired_paramend# Works as expected.example(42)# => 42# Should raise an error because no positional argument is being passed in,# instead it passes in an option hash to required_param.example(named_arg: 43)defexample2(required_param,second_required_param,**optional_named)# Some code...end# Should say it is missing two arguments, instead says it is missing one:example2(named: 42)

I understand that all of this is probably related to the option hash syntax leftover from when Ruby didn't have named arguments, but this all leads to very confusing code. Named arguments are much more powerful, easier to deal with, and more "Ruby-like" than the option hash syntax. As such I feel that Ruby should assume that if you use the ** operator in your parameters you never want that method to use an options hash.

In the distance future I even think that option hashes should be deprecated then removed, since named arguments do everything option hashes do and more. What opinions do you all have?

Sorry, I didn't see that issue. The cause is probably the same, but the difference is that #11860 deals with using the double splat in the argument list of a method call, whereas this issue deals using with the double splat in the parameter list of a method definition. That being said, these two issues can probably be merged.

without_parameters(*array)# => nilwithout_parameters(**empty_hash)# => nilwith_parameters(**empty_hash)# => []# This doesn't raise ArgumentError, because the method does not accept keyword# arguments, so a keyword splat is passed as a positional hash.with_parameters(**filled_hash)# => [{:example=>"value"}]with_one_parameter(**empty_hash)# ArgumentError (wrong number of arguments (given 0, expected 1))# This also doesn't raise ArgumentError, for the same reason as with_parameterswith_one_parameter(**filled_hash)# => {:example=>"value"}

There are no warnings in this code, and the behavior should be the same in Ruby 3.